Heretofore, there have been proposed a variety of hybrid vehicles each of which is mounted with an engine and an electric motor and which is so constructed as to drive a load by the motor, and some of these hybrid vehicles are in practical use.
For instance, there is proposed a hybrid vehicle in which a battery is charged with electric power generated from a generator by driving of an engine, and the electric motor is selectively driven either by electric power from the battery or electric power from the generator.
In the above-constructed hybrid vehicle, it is desirable to optimally control power distribution between the battery and the generator depending on electric power demanded from a load by way of the motor, considering various requirements such as securing stable operation of the engine, attaining output efficiency of the generator, and improving efficiency of the battery, and preventing deterioration of performance of the battery by suppressing generation of excessive charging/discharging currents.
As an apparatus for attaining the aforementioned power control, Japanese Unexamined Patent Publication No. 5-146008 proposes an arrangement in which an output terminal of a generator and an output terminal of a battery are respectively directly connected with a direct-current (DC) line, and output power from the generator is varied by controlling a field current of the generator in such a manner that the output voltage of the generator is set at a target value. With this arrangement, power distribution between the battery and the generator is controllably regulated.
Further, proposed is a hybrid construction machine loaded with an engine and an electric motor to drive an actuator by the motor. For instance, Japanese Unexamined Patent Publication No. 2000-283107 proposes a hybrid hydraulic excavator constructed such that power required for driving plural actuators is selectively supplied by way of the electric motor from a generator and a battery.
Generally, in a construction machine, particularly, in a hydraulic excavator for performing excavation by use of a working attachment, a load-driving torque which is to be outputted from an electric motor in order to drive an actuator (hydraulic cylinder) of the attachment is greatly varied depending on various factors such as a reactive force exerted from an object for excavation and the posture of the attachment, which resultantly increases a difference between a minimal load and a maximal load. Further, since the operating speed of each attachment is frequently adjusted by an operator, power required for driving a load is changed on time-basis. FIG. 8 illustrates an example of change of power demanded from a load with time at the time of excavating and charging by a hydraulic excavator. As is obvious from FIG. 8, power demand from a load is frequently and greatly changed between the minimal value and the maximal value.
In view of the above, even if the arrangement of the aforementioned Japanese Publication No. 5-146008 is applied to a hybrid vehicle in which power demand from a load is greatly changed, e.g. hybrid construction machine, the following matters should be considered because the above publication has the arrangement that the output terminal of the generator and the output terminal of the battery (storage device) are respectively directly connected with a DC line:
i) output distribution between the generator and the battery is determined based on a relation between a terminal voltage of the generator (battery), and an output impedance during a rise-up period until control keeps up with the change of the power demand; and
ii) the voltage and the current of the DC line greatly fluctuate depending on accumulated electric energy and the level of charging/discharging currents of the battery.
Due to the reason i), it is highly likely that the hybrid construction machine adopting the arrangement of the aforementioned publication may encounter an uncontrollable state if the power demand from the load is drastically changed. As a result, excessive power may be supplied from the generator, which may increase burden on the engine for driving the generator. Then, it is highly likely that fuel consumption rate or fuel efficiency of the engine may be lowered, or in a worst case, driving of the engine may be forcibly suspended owing to overload of the engine. Further, power loss due to internal resistance of the battery may be increased, or performance of the battery may be deteriorated owing to increase of charging/discharging currents of the battery.
Generally, means for driving a motor is electrically connected with a DC line. Considering a withstand voltage or a withstand current of a circuit element (e.g. semiconductor switches and diodes such as MOS-FETs or IGBTs) used in such motor-driver, it is required to adopt an element whose rated voltage is higher than the maximal value of the voltage of the DC line and whose rated current is regulated in accordance with the output current of the motor-driver.
Due to the reason ii), as the maximal voltage of the DC line increases, a circuit element having a high withstand voltage is required. As a result, the cost of the circuit element is raised, and the motor-driver becomes expensive. Further, if demanded power from the load is to be supplied in the above arrangement by way of the motor in an attempt to cope with a condition that the voltage of the DC line is lowered, the output current of the motor-driver is increased. In view of this, a circuit element having a large current capacity is required. Therefore, as with the former case, the cost for the circuit element is raised, which makes the motor-driver expensive. Furthermore, as the output current of the motor-driver increases, switching loss of a semiconductor switching element increases, which resultantly increases calorific power. Accordingly, the size of the motor-driver becomes large owing to increase of the size of cooling means such as a heat sink which is mounted on the semiconductor switching element. As a result, the motor-driver in its entirety becomes large, and a large space is required to load such a large motordriver in a hybrid vehicle.
In view of the above, it is an object of the invention to solve the aforementioned problems residing in the prior art and to provide a power control apparatus for use in a hybrid vehicle that enables to allow a generator and storage device to optimally supply power just enough for demanded power from a load even in the case where the demanded power from the load by way of an electric motor is drastically changed, and a hybrid construction machine loaded with such a power control apparatus.
It is another object of the invention to provide a power control apparatus for use in a hybrid vehicle that enables to improve charging/discharging efficiency of storage device, to prevent deterioration of performance of the storage device, and to downsize the power control apparatus, and a hybrid construction machine loaded with such a power control apparatus.